When used in the fields indicated above, laminated plastic material must possess a number of features the most important of which are fire resistance, strength, water resistance, thermostability, abrasive resistance of the surface, and provision of high-quality decorative facing.
Known in the art (cf. the USSR Inventor's Certificate No. 269,476, Int. Cl. B29a) is a laminated plastic of the following composition in weight percent:
______________________________________ paper for outer layers 21.0 paper for inner layers 37.0 phenol-formaldehyde resin 13.8 urea-formaldehyde resin 22.2 phosphoric acid 6.0 ______________________________________
The disadvantage of such plastic material evident in the process of its fabrication is that phosphoric acid used as a flame retardant material makes the process of impregnating papers with resins and that of drying the impregnated papers difficult. The disadvantage evident in application of the plastic of such a composition is its combustibility index K ranging from 1.2 to 1.3.
The fire resistance of laminated plastic is known to be dependent mainly on the presence of a flame retardant material in its composition, on the amount and chemical composition of the flame retardant material introduced, on the composition of binding agents, as well as on the thickness and components of the plastic.
The extent of fire resistance of a material might be evaluated by its combustibility index (K) using the calorimetric method.
The combustibility index (K) is determined as a ratio of the amount of heat released during the combustibility test to that obtained by the sample from the firing source during the test period.
Following such method, materials may be classified by fire resistance according to their K values as
______________________________________ combustible K above 2.1 hard-to-inflame K ranging of 0.5 to 2.1 hard-to-burn K ranging of 0.1 to 0.5 incombustible K below 0.1 ______________________________________
Under the conditions of increased fire hazard, a plastic material having a combustibility index of not above 0.5 should be used.
The extent of fire resistance ensured by the above method is directly dependent upon the plastic material's thickness. Thus the thickness decrease results in changes among the plastic components, i.e. number of the plastic inner layers becomes lower. Fireproofing of thin plastics is difficult to attain, however a decrease in thickness is necessary to lower the weight of construction details covered with laminated plastic material.
Strength of a plastic material is known to be determined by the value of its bending strength at failure, which should be no less than 1,000 kgf/cm.sup.2.
Water resistance of a plastic is known to be determined by the type of binding material used for protective layers, by the grade of paper impregnation, and by extent of the binding agent solidification in the process of pressing. Water absorption for a period of 24 hours usually is not above 4 to 6%, according to the thickness of the plastic material. In addition, a plastic material must withstand 1-hour boiling in water without formation of any swelling or stratification, thus featuring the grade of paper impregnation with the binding agent and the necessary extent of its solidification.
Thermostability and wear resistance of a plastic surface is known to be determined by the type of binding agent used for protective layers. Thus, the plastic based on melamine-formaldehyde resin is shown to have higher wear resistance and thermostability characteristics as compared to those of the plastic based on urea-melamine-formaldehyde resin, which makes it possible to classify it as hard-to-inflame.
Also known in the art is a laminated plastic material (cf. the book "Fireproofing of Wood Boards and Laminated Plastics" by A. A. Leonovitch et al. Moscow, Publishing House "Lesnaya Promyshlennost", 1974, pp. 98-115) composed of surface layers of paper, amino-formaldehyde resin and inner layers of paper including flame retardant material, and phenol formaldehyde resin.
Ferriammonium phosphate used as the flame retardant material in this type of laminated plastic enters into the composition of the paper for inner layers.
Indicated components enter into the composition of the laminated plastic in the following ratio (weight percent):
______________________________________ paper for surface layers 15 to 22 paper for inner layers 30 to 37 urea-melamine-formaldehyde resin 16 to 22 phenol-formaldehyde resin 20 to 25 flame retardant material 6 to 7 ______________________________________
Such a composition ensures a laminated plastic of 2 to 3 mm thick.
Disadvantages of the plastic laminate mentioned above are as follows. It has a combustibility index K ranging from 1.2 to 1.4, i.e. the plastic falls into the category of the hard-to-inflame materials. Therefore such plastic laminate is of limited application, in particular its use being unadvisable under the conditions of increased fire hazard. In addition, the above described laminated plastic cannot be provided with a thickness of below 2 mm in case its fireproof characteristics are to be retained, since otherwise inadequate efficiency of ferriammonium phosphate as the flame retardant material would result in a combustibility index of greater than 2.1 thus causing the plastic to fall into the category of combustible materials.